Barcelona sinkhole location found to be 'delicate' point in geological study 

Albert Vetayol, a member of the Georisk Observatory at Catalonia's College of Geologists, has attributed the sinkhole that opened in the Putxet neighborhood to overexcavation of the tunnel under construction for the Barcelona metro Line 9 (L9). 

The tunnel boring machine used to create the tunnel for L9 was advancing through granite terrain until it reached a point where the subsoil changed to slate, a much softer rock. 

According to Vetayol, this shift in rock type would have caused overexcavation, meaning the tools used would drag more material than expected and a cavity would then have opened up in the roof of the tunnel. This cavity would have eventually continued vertically and caused the collapse of the terrain at the surface. 

The geologist explained that geological surveys, tests and studies were carried out along the entire route of the L9, during which this location was identified as a "more delicate" area due to the change in rock. 

After the sinkhole opened, residents of 93 apartments in the Putxet neighbourhood were evacuated on Tuesday.

The crater, measuring around eight metres across, opened in an interior courtyard near the site of a future L9 station in the Sarrià-Sant Gervasi district.

Authorities established a 24-metre safety perimeter around the collapse, eventually evacuating eight residential buildings.

The cavity has been filled with concrete and the current protocols are "very safe" according to Lluís Moreno, the president of the Chamber of Public Works Contractors of Catalonia. 

Concrete injection is the usual system for consolidating this type of cavity, though some residents that were evacuated, expressed concern that three days is not enough time for the concrete to dry. 

Moreno assures that three days are sufficient for the concrete to fulfill its function. He continued to say that once the ground is stabilized, the work can continue with even more intense control.